Chlamydia trachomatis is the most common sexually transmitted bacterial pathogen in the world, causing serious complications on women's reproductive health including ectopic pregnancy, pelvic inflammatory disease and infertility. C. trachomatis also causes infection of the eye resulting in inflammation and in some cases blindness. The objectives of this project are to define the epidemiology, risk factors, transmission kinetics, and pathogenesis of C. trachomatis infections in different population settings, including populations in resource constrained countries. We have used the Internet site www.iwantthekit.org (IWTK) since 2004 to offer sampling in Maryland, the District of Columbia and Alaska for chlamydia screening in over 8,000 women and over 4,500 men using self-obtained vaginal swabs, penile-meatal swabs and rectal samples. Samples were also tested for gonorrhea and trichomonas. Prevalence of chlamydia for women overall was 6.3% and 7.7% in men. Both young age and Black race have been statistically associated with chlamydia positivity. Trichomonas prevalence in women was 6.1% and 2.2% in men. For rectal chlamydia in men, the overall chlamydia prevalence was 6.9% while for gonorrhea it was 4.0%. Acceptance for self-collecting penile-meatal and rectal swabs has been very high. Rectal chlamydia and gonorrhea prevalence in women was 6.3% and 0.9%, respectively. In 2004 chlamydia prevalence was 10.0% which has declined to 3.2% in 2017, an indication of a successful public health program. Recent data indicate that between 2013-16 positivity for any STI was 10.7% and 19.9% of STIs would have been missed if urogenital sampling only had been done (22.0% of male and 11.9% of female chlamydia infections.) To study the capability for performing a point-of-care self-test for trichomonas at home, we recently enrolled 102 women who performed the mailed point-of-care (POC) trichomonas test successfully at home. The test demonstrated high accuracy and high acceptability. Recently, as part of a consented-on-line study, 120 persons have requested and performed a POC HIV oral fluid test at home, with excellent satisfaction survey results. The IWTK website has over 10,000 page views per month. A pilot study (virtual PREP) has begun to add to add home collection of dried blood spots for syphilis and HIV testing for PREP monitoring for syphilis and HIV, as well as for chlamydia and gonorrhea. Trachoma due to C. trachomatis infection is the most common cause of infectious blindness in the world. The WHO has recommended that three rounds of mass drug administration (MDA) with antibiotics be offered to control the disease in districts where the prevalence of follicular trachoma (TF) is >10% in children aged 1-9 years, with treatment coverage of at least 80%. We have conducted both surgical and antibiotic treatment intervention studies in Gambia, Niger, and Tanzania in efforts to control trachoma. However, the source of infection following mass treatment is often unknown. If migrants into a village undergoing MDA are shown to impede progress towards elimination, then a local strategy that addresses treatment of new families and a nationwide strategy that addresses migration will be needed. The purpose of this study was to quantify the effect of migrants on the prevalence of infection and clinical trachoma in communities. In four communities in Kongwa, Tanzania, all children were enrolled in a longitudinal study of infection and trachoma. New children were identified at census updates as having not been in the community at the previous census. Within communities, neighborhoods were defined as spatially close groups of households. Children who were migrants were more likely to be infected and to have trachoma than children who were resident in the community, which was significant by the time of the survey following the third year of MDA (odds ratio, OR, 2.49, 95% confidence interval, CI, 1.036.05). The neighborhoods where newcomers resided were more likely to have infection a year later than neighborhoods with no migrants, which was most pronounced following the third year of MDA (OR 2.86, 95% CI 1.077.65). In summary, migrants to communities may be an important source of re-emergent infection, especially as MDA lowers infection among residents. We therefore conducted a community-randomized, clinical trial to determine if a surveillance program that targeted newcomers and travelers, identified weekly, would result in more communities achieving levels of infection of 1%. 52 communities were randomly allocated 1:1 to the control (annual mass drug administration with azithromycin (MDA) alone if warranted) or intervention arm (annual MDA if warranted, plus a surveillance program to identify and treat newcomers and travelers). In each community, surveys were completed every six months on a random sample of 100 children ages 1-9 years for trachoma and infection. The primary outcome was the proportion of communities in the intervention arm, compared to the control arm, which had a prevalence of infection at 1% by 24 months. Intervention communities experienced an average of 110 surveillance events per month. At 24 months, 7 (27%) of 26 intervention communities achieved a prevalence of infection 1% compared to 4 (15%) of the 26 control communities (odds ratio = 26, 95%CI = 056-119). At 24 months, the average infection prevalence in the intervention communities was 48, compared to 69 in the control communities (p = 06). Despite surveillance programs for community newcomers and travelers, the proportion of intervention communities with a level of infection 1% was lower than expected and not significantly different from control communities Recent work on a test for antibodies to C. trachomatis pgp3 antigens suggests serology is a promising tool that indicates cumulative risk of exposure to C. trachomatis. In a trachoma-endemic district in Tanzania that stopped its program four years ago, we undertook a surveillance survey, adding to the assessment of TF a lab test for C. trachomatis infection, and a dried blood spot which was processed for antibodies to C. trachomatis antigen pgp3; antibody status may indicate cumulative past exposure to infection. The prevalence of TF was 0.4%, below the 5% cut-off indicating that trachoma elimination had been achieved with no re-emergence. The antibody positivity overall was low, 7.5%, and increased with age from 5.2% in 13 year olds, to 9.3% in 79 year olds (p = 0.015). In 16 of the 30 hamlets, no children aged 13 years old had antibodies to pgp3. The antibody status of the 13 year olds indicated low cumulative exposure to infection during the surveillance period. In summary, four years post -program, there is no evidence for re-emergence of trachoma using any indicator sufficient to indicate re-emergence.